NEW YORK (CNN) -- Police said a teenager wrote in a journal, "I will start a chain of terrorism in the world," and outlined a plot to detonate a bomb and kill students at his high school in Bohemia, New York.

Long Island authorities arrested a 15-year-old and a 17-year-old identified as Michael McDonough, Friday on charges of conspiring to attack staff and students.

Police said McDonough and the 15-year-old worked at a McDonald's, where the journal was discovered by an adult in the parking lot.

The journal was passed to school administrators July 6.

Officials said the 15-year-old, who was on a long-term suspension from the school in Bohemia, is believed to be the primary author of the journal. McDonough, who attended a Long Island high school, was set to help carry out the alleged plans, authorities said.

Suffolk County Police Commissioner Richard Dormer said the journal described shooting students and setting off homemade explosions.

Police said a search warrant was issued for the 15-year-old's computer, where it was discovered that he'd made numerous attempts and inquires on the Internet to obtain weapons, including an Uzi and five pounds of black powder explosive.

Police found several videotapes in which the 15-year-old appeared. Dormer described them as "chilling."

He also said the plans were very elaborate. One of the videotapes was at least 10 minutes long.